Correct examples in 'Changing DocumentRoot'

[No5251]

Replace single quotes with double quotes so that the variable is replaced

[yosifkit]

The single quotes are on purpose since Apache httpd will evaluate them in the config at runtime. This allows extra ability to change the root at runtime as well setting a default during a custom build.

From the lines explaining this example:

The following demonstrates one way to do so using an environment variable (which can then be modified at container runtime as well):